This application is for the second year continuation of a previously awarded grant to support a multidisciplinary program of basic research on the viral etiology of cancer. Studies during the past year have confirmed that breast cancer in mice can be prevented by prophylactic immunication and additional evidence has confirmed that there is a human mammary tumor virus and it may be related to the mouse mammary tumor virus (MTV). Additional studies will seek to isolate and characterize all fractions of the human and mouse mammary tumor viruses using the techniques of biochemistry, molecular biology, immunology, cytogenetics, biophysics, cell culture and microbiology.